The present invention relates to a process of producing a titanium oxide coating agent for the purpose of forming a titanium oxide film on a substrate, and a titanium oxide film coating formed using such a coating agent.
Among processes of forming a titanium oxide film, there are a coating process wherein a titanium oxide powder slurry or an aqueous solution of titanium chloride is coated and then fired on a substrate, a sol-gel process wherein a sol prepared by hydrolysis of a metal alkoxide is coated and then fired on a substrate, a sputtering process wherein an oxide target is sputtered in a high vacuum to form a film on a substrate, a CVD process wherein an organometallic compound or a halide is decomposed upon volatilization in a heating furnace to form a film on a substrate, a plasma spray coating process wherein solid particles are fused by a plasma generated in the atmosphere to spray them onto the surface of a substrate, etc. Of these processes, the process using a coating solution is now considered to be simple and of high practicality for the formation of a titanium oxide film.
With the titanium oxide powder coating process that is simple, however, it is difficult to obtain an intimate film in close contact with the substrate. Generally, high synthesis temperatures are needed for titanium oxide film formation, and so it is required to use a heat-resistant substrate capable of standing up to such synthesis temperatures. Thus, there is some limit to the type of available substrate.
Another problem with this process is that harmful halogen compounds, etc. are generated by firing because acids or suitable organic dispersants are commonly used to obtain a dispersion solution of titanium oxide fine particles. In the process of coating and firing an aqueous solution of titanium chloride, titanium sulfate, etc., too, harmful halogen compounds are generated. In addition, firing temperatures as high as several hundred 0xc2x0 C. are needed.
A commercially available titanium oxide sol prepared by the sol-gel process is industrially advantageous in that it can be coated and impregnated, coated over a large area and synthesized at low temperatures. One problem with this sol is, however, that the raw materials are not only expensive but also chemically unstable and susceptible to influences by temperature control and atmospheres and so hard to handle, because the synthesis must be carried out using organic metals such as titanium tetraisopropoxide and tetrabutyl titanate.
Another problem with the sol-gel process is that it is unsuitable for materials susceptible to attacks by acids because heating at 400xc2x0 C. or higher is needed for removal by firing of acids and organic substances contained in the starting sol. Low-temperature firing is likely to yield a porous product.
Yet another problem with the sol-gel process is that it involves complicated process steps and has to use an organic solvent. A titanium oxide sol prepared by the sol-gel process contains acids and alkalis or organic substances, and so offers a corrosion problem with respect to the substrate material to be coated. A further problem is that temperatures of at least 400xc2x0 C. are needed for the decomposition of the organic substances, and so harmful by-products such as halides and nitrogen oxides are generated during firing by heating.
With the prior art processes, it is thus difficult to prepare a crystalline titania film of high density at low temperatures. For the sol-gel process capable of preparing the titania film at a relatively low temperature, on the other hand, the organic substances, acids, etc. must be decomposed and removed by thermal treatment. Otherwise, this makes the titania film porous; that is, the thermal treatment temperature should be relatively high so as to prepare a film of high density. Besides, such aids are unfavorable because harmful substances such as nitrogen oxides and organic substance vapors are generated by the thermal treatment.
When a titanium oxide film is formed of a peroxotitanium hydrate, on the other hand, it is known that a film having good properties can be obtained at a relatively low temperature. It is also known that the peroxotitanium hydrate is formed by direct addition of an aqueous solution of hydrogen peroxide to a solution of titanium tetrachloride, titanium sulfate or the like thereby forming peroxotitanium hydrate ions, and permitting the hydrate ions to precipitate out in a solid form.
At pH 1 or higher, the peroxotitanium hydrate ions are also generated in the form of polynuclear ions containing at least two titanium atoms. At normal temperature, these hydrate ions condense slowly and then precipitate out. It is thus difficult to use the peroxotitanium hydrate ions at pH 1 or higher as a titanium oxide coating agent; in other words, there is some limit to the type of substrate to which a strong acid coating agent of pH 1 or less can be applied. In addition, harmful substances such as hydrogen halides and sulfur oxides are generated by thermal treatment from halogens, sulfur, etc. included in the hydrate ions.
To prepare a titanium oxide film of high purity, JP62-252319(A) has proposed to add a hydrogen peroxide solution directly to hydrogenated titanium or alkoxytitanium for dissolution, thereby producing peroxidized titanium, i.e., a substance regarded as a peroxotitanium hydrate.
A problem with these titanium raw materials is, however, that their unstableness causes some considerable exothermic reaction when the hydrogen peroxide solution acts thereon, resulting in adverse influences such as thermal decomposition of the raw materials and the product. When the peroxotitanium hydrate is produced in large amounts, therefore, the resulting peroxotitanic acid polymerizes and increases in viscosity. Worst of all, particles grow to such an extent that the transmission of light through the solution is cut off and so the solution becomes turbid. When the solution is used as a coating agent, this in turn causes the close contact of the film with an associated substrate to become worse and the density of the film to drop as well.
JP63-35419(A) and JP01-224220(A) disclose a process of producing an aqueous solution referred therein to as a titanyl ion hydrogen peroxide complex or titanic acid and regarded as a peroxotitanium hydrate by adding a hydrogen peroxide solution to a hydrous titanium oxide gel or sol.
Upon the direct addition of the hydrogen peroxide solution to titanium hydroxide, however, much heat is generated due to the simultaneous occurrence of peroxidization and solution formation and, hence, sufficient cooling under agitation is needed. However, as the amount of the peroxotitanium hydrate to be produced increases, temperature control becomes difficult. Unless sufficient cooling can be carried out, a polymer grows in the form of particles due to viscosity increases and condensation. This may in turn cause the solution to become turbid.
When a gel or sol of hydrous titanium oxide is prepared, it is common to add a basic substance such as ammonia thereto. However, impurities, i.e., cations such as ammonium ions and anions such as chlorine ions are likely to be taken in and absorbed on the gel or sol due to momentary precipitation of hydrous titanium oxide. In particular, the presence of anionic impurities such as chlorine ions and sulfate ions may often promote condensation of a peroxotitanium hydrate formed after the addition of a hydrogen peroxide solution, resulting in a failure in obtaining a transparent aqueous solution. In addition, complete removal of impurities is difficult to achieve even when the hydrous titanium oxide is washed with distilled water. In view of stable production of peroxotitanium hydrate, there is thus a grove problem.
In Japanese Patent Nos. 2875993 and 2938376, the inventors have already showed that a coating agent with anatase ultrafine particles dispersed can be obtained by heating an aqueous solution of peroxotitanium hydrate, thereby making it possible to form a crystalline titania film of improved adhesion.
With the processes already proposed by the inventors, it is possible to form a crystalline titania film that is more improved in terms of adhesion than that obtained by a conventional process. However, when an anatase sol is prepared by heating an aqueous solution of peroxotitanium hydrate while the amount of cation residues such as ammonium ion residues is large, peroxo groups are less susceptible to decomposition, yielding large particles. When these particles are used as a coating agent, a problem often arises in conjunction with adhesion or density.
Especially when anionic impurities such as chlorine ions and sulfate ions remain in a large amount, the polymerization of peroxotitanic acid formed after the addition of a hydrogen peroxide solution is often promoted, resulting in a failure in obtaining a transparent aqueous solution or a density increase. Even with a titanium hydrate with ionic impurities adsorbed thereon, it is prima facie possible to prepare a transparent solution of peroxotitanic acid if a hydrogen peroxide solution is allowed to act thereon after the concentration of ionic impurities is reduced by repeating washing with purified water. However, too long a time is needed for washing because as the lower the impurity concentration, the more difficult it is to precipitate the hydrate.
It is thus strongly desired to provide a production process that can be more easily carried out than a process using the titania film-forming solution proposed by the inventors in Japanese Patent Nos. 2875993 and 2938376 and provide a solution enabling a stable titania film of improved properties to be obtained even when produced in large amounts.
Accomplished to eliminate such problems as explained with reference to the prior art, the present invention provides a new process of producing an aqueous solution of peroxotitanium hydrate. In other words, it is one object of the present invention to prevent condensation due to thermal influences or growth of particles than needed in the process of producing an aqueous solution of peroxotitanium hydrate with a hydrogen peroxide solution, which may otherwise cause a film adhesion or density drop when the aqueous solution is used as a coating agent. It is another object of the present invention to obtain a titanium oxide-forming solution that makes it possible to form a titanium oxide coating film of improved adhesion and increased density.
The present invention also provides a new process of producing a crystalline titanium oxide particle that is also useful as a titanium oxide-forming film coating agent. In other words, it is one object of the present invention to prevent condensation due to thermal influences or growth of particles than needed in the process of producing an aqueous solution of peroxotitanium hydrate with a hydrogen peroxide solution, which may otherwise cause a film adhesion or density drop when the aqueous solution is used as a coating agent. It is yet another object of the present invention to obtain a crystlline titanium oxide particle that makes it possible to form a titanium oxide coating film of improved adhesion and increased density.
It is a further object of the present invention to provide a titanium oxide-forming solution usable as a titanium oxide coating agent or the like, and a process of producing the same. It is a further object of the present invention to provide a stable titanium oxide-forming solution with which problems with a conventional process of producing titanium oxides can be solved, and a new process of producing a sol with anatase fine particles dispersed therein, which can be obtained from the same. It is a further object of the present invention to provide a production process wherein solution formation can be promoted by adjustment of impurity ions in the reaction step at which a titanium oxide-forming solution is obtained from the starting materials.